This invention relates to medical ultrasonic imaging, and in particular to methods for improving the imaging of contrast agents.
The nonlinear response of contrast agents is a unique property that improves the detectability of contrast agents in the blood and/or tissue. At the second harmonic, third harmonic, sub-harmonic, or other harmonics, the agents effectively generate signals that are unique to contrast agent. Since the contrast agent response to the incident sound wave is not linearly related to the impinging sound pressure, nonuniform peak pressure fields can generate nonuniform images of contrast agents that are much more nonuniform than corresponding tissue images at the fundamental frequency. Nonuniform images reduce diagnostic confidence. Even linear imaging of contrast agents or the combination of linear imaging with nonlinear imaging can produce nonuniform images due to inhomogeneous pressure fields.
For the detection and characterization of lesions in the liver and spleen, contrast agents such as Levovist are especially useful since they accumulate in healthy tissue to a greater extent than in diseased tissue. The imaging of these agents often shows dark voids where the lesions are detected, provided the peak pressure is sufficient to generate a strong nonlinear response from the contrast agent. With some imaging techniques, an agent such as Levovist generates a high-contrast image, where detected agent is shown much brighter than the surrounding contrast-free tissue or undetected (or poorly detected) agent. Nonuniform peak pressures in space can limit the area over which the contrast agents are detected, and low-brightness areas become suspicious areas and often need to be re-scanned.
Some contrast agents are preferably imaged using a low transmit peak pressure or low mechanical index (MI), since this reduces agent destruction, maintains agent concentration, and prolongs enhancement. Other contrast agents are preferentially detected by significant disruption or destruction with high transmit peak pressures. The conventional beamforming techniques (focussing transmitted pressure waves through time delays and phasing adjustments) produce nonuniform pressure fields as a function of depth. This reduces the area of good image enhancement at low as well as high transmit peak pressures.
A technique that more uniformly distributes peak acoustic pressure over a given depth will increase the area over which contrast agents can be used to aid diagnosis. For some contrast agents and imaging situations, increasing the peak pressure above a pressure threshold may not produce noticeable increases in agent detectability. Techniques that can generate peak pressures above a threshold over a greater spatial extent, even with a possible reduction in maximum peak pressure in some areas, can produce improved images of contrast agents.
As described in detail below in conjunction with the preferred embodiments, the present inventors have discovered that transmitted plane waves, or wavefronts that are substantially planar can generate peak pressures that are more uniform over depth, and this improved uniformity can improve the detectability of contrast agents. Depending upon the type of contrast agent being used, the returned frequencies of interest (e.g., fundamental, harmonic, selected harmonic orders, or a combination of fundamental and one or more harmonic orders) and the desired strength of the nonlinear response, multiple substantially planar wavefronts can be generated at substantially the same time to increase peak pressures.
The foregoing remarks have been provided only by way of introduction, and they are not intended to narrow the scope of the following claims.